Premature crosslinking, which is called scorching, during the preparatory phase is a major difficulty in using peroxides in crosslinking (curing) applications of elastomeric and/or thermoplastic materials. The preparatory phase consists in general in blending the constituents and optionally extruding them, often at high temperatures. The operating conditions for this preparatory phase lead very often to the partial decomposition of the peroxide initiator, thus inducing the premature crosslinking reaction with formation of particles of gel within the mass of the mixture. The presence of these gel particles leads to imperfections (inhomogeneity, surface roughness) in the final product. Excessive scorching reduces the plastic properties of the material, such that it can no longer be transformed, which leads to the loss of the whole batch. In addition, excessive scorching can lead to the complete stoppage of the extrusion operation. Moreover, the organic peroxide scorching time obliges the user to carry out the blending at low temperatures, which imposes long mixing times.
To overcome this disadvantage, several solutions have been proposed. Thus, it has been proposed to use an initiator whose half-life time is long. The disadvantages of this approach are the low productivity due to a long curing time and the high energy costs.
It has also been proposed to incorporate certain additives in order to reduce the tendency towards scorching. Thus, the use of organic hydroperoxides as scorching inhibitors for compositions based on poly-ethylene crosslinked with a peroxide has been described in British patent GB 1 535 039. The use of vinyl monomers has been the subject of U.S. Pat. No. 3,954,907. The use of nitrites has been described in U.S. Pat. No. 3,202,648. U.S. Pat. No. 3,335,124 describes the use of aromatic amines, phenolic compounds, mercaptothiazole compounds, sulfides, hydroquinones and dialkyl dithia-carbamate compounds.
The use of the additives of the state of the art cited above to extend the time for withstanding scorching has however a harmful effect on the curing time and/or the final crosslinking density. It therefore leads to a decrease in the productivity and/or the properties of the final product.
In patent EP 0837 080, a process is described for grafting a functional monomer, in particular maleic anhydride, onto a thermoplastic polymer in the presence of a nitroxide, the role of which is to avoid crosslinking during the grafting operation.
International application WO 02/28950 describes scorch-retarding compositions comprising a nitroxide containing at least one unsaturation and an organic peroxide. The mass ratio of organic peroxide to nitroxide is between 1 and 50.
In the international application WO 02/28946, scorch-retarding compositions comprising a nitroxide, a crosslinking promoter and an organic peroxide are described.
In Japanese patent application JP 11-49865, additives of the nitroxide family have been described in compositions for retarding scorching during the crosslinking of polyethylene. These nitroxides correspond to general formula I:
in which R1, R2, R5 and R6 represent an alkyl group of 1 to 4 carbon atoms and R3 and R4 represent an alkyl group of 1 to 4 carbon atoms or are linked and represent a group —CH2—CHX—CH2—, it being possible for X to be a hydroxyl, methoxy, cyano, phenylcarbonyloxy, carboxyl or methoxycarbonyl group.
The compounds 2,2,6,6-tetramethyl-1-piperidinyloxy (called TEMPO) and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (called HTEMPO) are preferred because of their marked effect on the prevention on scorching.
The nitroxides of formula I are used in the form of a mixture with an organic peroxide. Among the 20 or so organic peroxides cited, dicumyl peroxide (also called DCP) and bis(α-tert-butylperoxyisopropyl)benzene in combination with a compound of general formula I are preferred as scorch-retarding agents.
The organic peroxide and the compound of general formula I are used in weight ratios ranging from 1:0.02 to 1:1 and advantageously from 1:0.1 to 1:0.5. Outside these ratios, if the content of compound of general formula I in terms of peroxide is less than 1:0.02, the scorch-reducing effect is less marked; if it is greater than 1:1, then an appreciable reduction in the cross-linking density is noted. The ratio by weight of organic peroxide normally represents between 0.3 and 5 parts per 100 parts by weight of polyethylene, and advantageously between 1 and 3 parts.
Thus, Table 1 of patent application JP 11-49865 shows that the addition, to 500 g of low-density poly-ethylene, of 12.5 g of DCP with 2.5 g (0.5%) or 5 g (1.0%) of TEMPO or HTEMPO makes it possible to increase the scorching time at 145° C. expressed in the form of time A (time necessary to reach 10% of the maximum pair) and time B (time necessary to reach the pair value of 0.4 kgf.cm), compared with the addition of 12.5 g of DCP alone.
Conversely, the addition, to 500 g of low-density poly-ethylene, of 12.5 g of DCP with 2.5 g (0.5%) or 5 g (1.0%) of TEMPO or HTEMPO, causes a very marked reduction in the crosslinking density expressed in the form of maximum pair value at 180° C. compared with the addition of DCP alone.
Although the use of these compounds during crosslinking brings about an extension of the scorch-resistance time, the reduction in the final crosslinking density represents a major disadvantage. Indeed, the cross-linking density is an indication of the mechanical properties of the final article. Thus, a reduction in the crosslinking density most probably indicates a reduction in the mechanical properties. It may be envisaged, in some cases, to compensate for this reduction in crosslinking density by an increase in the quantity of crosslinking system (peroxide), at the expense of the economic aspect and of the formation of larger quantities of decomposition products.